A fundamental knowledge of the role of nitrogen on phase transformations remains a key point for mastering thermochemical treatments as carbonitriding for automotive applications. Carbonitriding consists in three main steps: enrichment in carbon and nitrogen in the austenitic field, quenching and tempering. The mastering of phase transformations and microstructural evolutions during cooling and tempering is determinant for controlling the final mechanical properties. The effect of nitrogen on the decomposition kinetics of the austenite during cooling and the resulting microstructures have been studied thoroughly previously for a low alloyed steel(23MnCrMo5). The aim of this work, is to analyse the role of nitrogen during tempering. Indeed, the tempering of nitrogen martensites has been studied in literature mainly for model systems and there is a lack of knowledge for multiconstituent industrial steels. For our experimental approach, we use 23MnCrMo5 steel samples homogenously enriched with carbon and nitrogen (ranging from 0.23 % wt to 0.63 % wt for carbon and from 0 % wt to 0.5 % wt for nitrogen for being representative of the compositions of carbonitrided layers) in austenite at 900°C and quenched. Continuous heating (at 1°C/s) and isothermal tempering treatments at different temperatures (160 °C, 300 °C and 400 °C) are then studied by means of dilatometry, high energy x-ray diffraction (HEXRD) and (S)TEM combined with EDX and ASTAR.